feat: integrate PowerContext with ZCode CLI and Windows desktop

[jackie-cqz]

Feature description

Add a PowerContext integration for the open-source ZCode CLI and the official Windows desktop edition. Users should be able to install one ZCode plugin that captures conversation Sources, prepares context for later turns, and exposes PowerContext MCP tools.

Problem and proposed solution

ZCode users currently lack a documented, repeatable path from a normal prompt to durable PowerContext memory and recall in a fresh session. Provide a plugin with a UserPromptSubmit Hook and MCP server declaration, plus powercontext setup zcode and powercontext doctor zcode. Setup should preserve unrelated ZCode configuration and keep the Hook and MCP endpoint on the same Server URL. Authentication should come from the ZCode process environment rather than stored plugin credentials.

Acceptance criteria

• Install and diagnose the plugin with an open-source CLI build or the official Windows desktop edition.
• Capture an ordinary prompt as a Source, process it into Memory when Generation is configured, and recall it in a fresh session.
• Expose MCP Memory and Handoff operations, including a first Handoff whose nullable carrier fields round-trip through prepare, continue, and commit.
• Keep ordinary ZCode turns usable during a Server outage and recover after the Server returns.
• Document setup, local Bearer authentication, troubleshooting, and the scope of real-host validation in English and Chinese.

Additional context

The planned implementation lives on a local feat/zcode-integration branch. Real-host validation has covered official Windows desktop ZCode 3.14.3 against a local PowerContext Server, including a clean user-owned profile. Cross-machine HTTPS connectivity and other official desktop versions still need separate validation.